1. Field of the Invention
The present invention relates to a method for reducing a torque ripple of a Switched Reluctance Motor (SRM) and particularly, to a method for reducing a torque ripple of a SRM, which detects the position of a rotor of a motor using a position detection sensor and reduces the torque ripple of the motor by designing the optimum pulse width of the position detecting signal and adjusting the duty rate of the pulse width modulation signal.
2. Description of the Background Art
Generally, a position signal of a motor rotor is required to drive a SRM (hereinafter as motor) and the motor can be controlled in the normal/reverse direction by recognizing a normal/reverse turning point of rotation accurately with a minimum one position sensor.
FIG. 1 is a block diagram showing a motor in accordance with the conventional art. As shown in the drawing, a conventional motor is comprised of three position detection sensors 301, 302 and 303 for detecting position of a motor rotor, pulse width modulation signal 110, a main controlling unit 100 for controlling three phase signals 120, 130 and 140 and the position detection sensors 301, 302 and 303, a motor driving unit 50 for inputting the three phase electric current to the motor by the three phase signal inputted from the main controlling unit 100, and a motor 200 driven by the three phase electric current inputted from the motor driving unit 50. Reference numerals 115, 125 and 135 designate AND gates.
FIG. 2 shows respective wave form charts of signals outputted from FIG. 1 and FIG. 3 is a wave form chart of a torque ripple in accordance with the conventional art.
With reference to FIGS. 1, 2 and 3, description of driving of the motor according to the conventional art is as follows.
The position detection sensors 301, 302 and 303 input the position detection signals 150, 160 and 170 to the main controlling unit 100 by detecting the rotor position of the motor. According to the detection result of the sensors the pulse width modulation signal 110 outputted from the main controlling unit 100 and the three phase signals 120, 130 and 140 are computed by a logical AND operation and then inputted to the motor driving unit 50. The motor driving unit 50 inputs electric current into each phase according to the signals inputted from the main controlling unit 100.
Wave forms of respective signals according to rotation of the motor 200 are shown in FIG. 2. Firstly, in the wave forms of respective phases, the position detection signal is on for a certain time, for example, a time duration that the rotor of motor rotates fifteen degrees of a mechanical angle (hereinafter the mechanical angle will be omitted) whenever the respective sensor in a rising edge of respective detection signals and the respective signals are inputted to the motor driving unit after performing logical AND operation with the pulse width modulation signal 110. The calculated signals controls the three phase electric current of the motor driving unit and electric current of respective phase of the motor driving unit 50 is inputted to the motor 200.
Namely, if the first sensor 301 is turned on, A phase signal of 120 in the rising edge of the first position detection signal 150 becomes ON. The signal of phase A is inputted to the motor driving unit 50 after performing a logical AND operation with the pulse width modulation signal 110 and the motor driving unit 50 inputs the A phase electric current to the motor according to the control of the signal. At this time, the motor 200 starts to rotate and after a certain time duration (for example, a time that the motor rotor rotates fifteen degrees) and if the second sensor 302 is turned on, the B phase signal 130 is turned on in the rising edge of the second position detection signal 160. The B phase signal 130 is inputted to the motor driving unit 50 after performing logical AND operation with the pulse width modulation signal 110 and the motor driving unit 50 inputs the B phase electric current to the motor 200 according to control of the signal.
Later, if the second sensor 302 is turned off, the A phase signal 120 becomes OFF in a falling edge of the second position detection signal and if the third sensor is turned on after a certain time period, a C phase signal 140 in the rising edge of the third position detection signal 170. The signal is inputted to te motor driving unit after performing a logical AND operation with the pulse width modulation signal 110 and the motor driving unit 50 inputs the C phase electric current to the motor according to the control of the signal.
If the third sensor 303 is turned off, the B phase signal 130 becomes OFF in the falling edge of the third position detection signal. If the first sensor 301 is turned on after a certain time period, the A phase signal 120 becomes ON in the rising edge of the first position detection signal 150 and the C phase signal 140 becomes OFF in the falling edge of the first detection signal.
According to the above process, signals of respective phase signals and the pulse width modulation signal 110 are inputted to the motor driving unit 50 after performing logical AND operations respectively. In addition, the motor driving unit 50 inputs electric current of each phase to a stator coil and the motor 200 rotates in the above pattern.
At this time, a torque which is a sum of respective torques is generated by the three phase electric current inputted to the motor and a torque ripple as shown in FIG. 3 is generated. In FIG. 3, a horizontal axis designates a rotational angle and a vertical axis designates a size of the torque ripple.
The driving method of the motor in the conventional art has a disadvantage that much amount of noise is generated due to much amount of the torque ripple.
Therefore, an object of the present invention is to provide a method for reducing a torque ripple of an SRM for detecting a position of the rotor using a position detection sensor, designing a pulse width if a position detection signal as an optimum one and decreasing the torque ripple of the motor by adjusting a duty rate of the pulse width modulation signal.
To achieve these and other advantages and in accordance with the purpose of the present invention, the present invention comprises the steps of setting a pulse width of a position detection signal in accordance with a position detection result of a motor rotor, outputting a signal for controlling of each phase in synchronization with a rising and falling edge of the position detection signal, and varying and outputting a duty rate of a pulse width modulation signal from the moment that a falling edge of the position detection signal is detected.
The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.